1. Field of the Invention
The present invention relates to an upconverter arrangement for CO.sub.2 lasers and, more particularly, to an upconverter arrangement which utilizes anti-Stokes Raman scattering in alkali atoms to upconvert CO.sub.2 lasers to the near UV or visible spectral regions. In accordance with the present invention, efficient CO.sub.2 lasers may be Raman shifted in one step to the 300 nm to 400 nm spectral region.
2. Description of the Prior Art
An anti-Stokes Raman laser may be defined as stimulated anti-Stokes Raman emission induced by a pump laser between two levels of the same parity in which a population inversion exists between the upper and lower Raman states. Such laser devices are particularly attractive since they are tunable by tuning the pump laser and, because the upper Raman state is often a metastable level, large inversion densities and high anti-Stokes output energies are possible. Early work in this area is reported in an article entitled "Observation of Stimulated Anti-Stokes Raman Scattering in Inverted Atomic Iodine" by R. L. Carman et al appearing in Physical Review Letters July 22, 1974 at pp. 190-193. As described therein, measurable gain in inverted I atoms may be obtained, where the I*(5p.sup.5 2.sbsp.P.sup.o.sub.1/2) state is populated by flash photolysis of trifluoromethyliodide (CF.sub.3 I). The anti-Stokes Raman signal may be observed by pumping this inversion with the fundamental of a Nd:YAG laser at 1.06 .mu.m and probing with a broadband dye. The article goes on to state, however, that superfluorescent emission at the nonresonant anti-Stokes wavelength was not observed during these experiments.
Observation of tunable, stimulated, vacuum-ultraviolet anti-Stokes Raman emission was later reported in the article "Tunable, 178-nm Iodine Anti-Stokes Raman Laser" by J. C. White et al appearing in Optical Letters Vol. 7, Nol 5, May 1982 at pp. 204-206. As reported, a metastable I* population inversion was created with respect to the ground site by selective photodissociation of NaI. With a 206 nm pump laser to drive the Raman process, anti-Stokes Raman laser radiation at 178 nm was generated with a pulse energy of 35 microjoules and was tunable over 10 cm.sup.-1.
One arrangement which is capable of achieving Raman shifting is disclosed in U.S. Pat. No. 4,144,464 issued to T. R. Loree et al on Mar. 13, 1979, which relates to a device and method for nonresonant Stokes Raman shifting of ultraviolet radiation. As disclosed, Stokes Raman shifting of broadband UV excimer laser radiation is achieved by varying the pressure of the Raman scattering medium, the focal interaction length of the incident radiation within the Raman scattering medium, and its power density level. Gaseous molecular H.sub.2,D.sub.2,CH.sub.4, HD and mixes thereof, and liquid N.sub.2 are used as the Raman scattering medium to frequency shift the outputs of high power KrF and ArF lasers.
An alternative arrangement is disclosed in U.S. Pat. No. 4,151,486 issued to I. Itzkan et al on Apr. 24, 1979, which relates to a tunable alkali metallic vapor laser system. Stimulated Stokes Raman scattering in a low pressure atomic alkali metallic vapor of potassium or cesium is utilized wherein the atomic vapor is provided and them primed to populate an intermediate level such as the 4P level for potassium and the 6P level for cesium from which the desired upper laser level is accessible. After the population is created at the intermediate level, it is pumped to a virtual level near the desired upper laser level by a tunable dye laser and, via the stimulated Raman scattering process, generates the tunable output laser beam.